A number of years ago "superabsorbent materials", i.e., materials which will absorb many times their weight of liquid were developed. Currently, the superabsorbent materials used in industry are synthetically formed compositions, such as polyacrylates. It is the latter class of compositions derived from acrylic acid to which the present invention pertains.
Polyacrylic acid compositions have been known for some time as is evidenced in the U.S. Pat. No. 2,833,745. This patent teaches preparing the alkali metal or ammonium salt of a polyacrylic acid by mixing the acrylic acid with alkali, or ammonium carbonates, or bicarbonates and then carrying out the polymerization by the addition of accelerators in a completely anhydrous medium or in a small amount of water. The polymerizate obtained is solid and somewhat porous. The solid product is then comminuted to a powder. The resulting polymer product is soluble.
In the series Textile Science and Technology published by Elsevier Science Publishers B.V., in Volume 7, entitled "Absorbency", (.COPYRGT.) 1985, Chapter VI discusses the Development of Synthetic Superabsorbent Polymers, including cross-linked polyacrylate superabsorbents, referenced to various patents and other literatures.
The superabsorbent polymeric compositions of the present invention differ from those of the prior art, not in their main monomeric constituents, but in the fact that the acrylic acid monomer is substantially fully neutralized and that the residual monomer level is extremely low.
The presently commercially available superabsorbents contain residual monomer levels as high as 4,000 ppm (parts per million). The industry has been trying to eliminate or at least significantly lower these levels, without substantially increasing the price. Arakawa currently sells a superabsorbent with residual monomer levels of 400 ppm. The Arakawa material is made by a different process than the present invention, and has a different composition in that it is not substantially fully neutralized. As shown by various tests, it and other prior art commercial superabsorbents have lower pH values, and are neutralized to a lesser degree than the superabsorbents of the present invention. By the use of the novel process of the present invention, this industry goal of moving closer to eliminating any monomer residue has been approached when using the most preferred materials, and great improvement has been made even with the less preferred embodiments.
Various different types of absorbent compositions are taught in U.S. Pat. No. 4,043,952, which treats the surface of a water absorbent composition by ionically complexing the surface of the composition. The water absorbent composition is dispersed, with a polyvalent metal cation, in a dispersing medium in which the composition is insoluble. The composition is an anionic polyelectrolyte. The dispersion is maintained at a given temperature for a period of time sufficient to ionically complex the exposed surface of the composition.
In the U.S. Pat. No. 4,090,013 a water-swellable, water-insoluble absorbent composition is prepared which has the ability to uncomplex at an elevated pH and recomplex at a lower pH. Although, as evidenced by the above, the water-swellable, water-insoluble compositions have been treated to improve their characteristics, no single-step process has provided a water-swellable, water-insoluble composition which absorbs water rapidly, has a low monomer content, and a low residual acid content.
Foamed water-swellable polymeric water absorbent materials are taught in Scott et al., U.S. Pat. No. 4,529,739 (assigned to Dow Chemical) including alkali metal salts of homopolymers of acrylic acid and certain copolymers thereof, which are made by contacting a latex dispersion of a polymer with a carbon dioxide yielding decomposeable blowing agent capable of neutralizing pendant moieties (e.g. sodium carbonate and sodium bicarbonate, among others). Applicants process involves some of the same materials, but applicants neutralize a monomer with (bi)carbonates while the Scott et al. process neutralizes a polymer.
Microwave Heating (ultra high frequency) radiation has previously been used for heating and/or drying and even for effecting chemical modification of starch (EPO Pat. No. 0,041,316 to CPC International) but it has not been used for the purpose of the present invention to provide an environment permitting uniform heating and reacting throughout a chemically initiated polymerization reaction, to the best of our knowledge.